1. Field of the Invention
The invention relates in general to the thermomechanical manufacturing of dense articles from powders and, in particular, to a method of forming dense articles from less dense preformed bodies of powders wherein the preformed bodies are plasticized and compressed by a process of frictional heating.
2. Description of the Prior Art
The use of thermomechanical processes to bond or weld articles together and to form coatings is well known. The use of powder metallurgy to form net or near-net shape parts is also well known.
Heat generated by moving materials relative to one another under pressure had been previously proposed for the plasticizing of powdered materials so as to form extruded articles from those powdered materials. See, for example, Thomas et al. U.S. Pat. No. 5,262,123. Thomas et al. propose to plasticize powdered materials by frictional heating, and to extrude the resulting materials. The loose powdered materials may be confined in cavities in a consumable rotating probe, or confined between non-consumable counter-rotating members. In the proposed configuration where the loose powdered materials are confined in cavities within a consumable probe, the material of the probe is mixed with the loose powdered materials as they are plasticized, and the resulting mixed material is extruded. In the configuration where loose powdered materials are confined between non-consumable counter-rotating members, the loose powders are confined by some other supporting structure, such as a chamber, until the plasticized mass is extruded. Unconfined loose powders can not be utilized according to Thomas et al. Where a consumable probe confines powdered metal therewithin, the probe must have sufficient structural strength to withstand the loads applied by moving it and simultaneously urging it into engagement with an opposing member. Also, the material of which the probe or rod is composed must be a desirable component in the final article, because it will inevitably be there. Thomas et al do not utilize unconfined loose powders. The applicability of the teachings of Thomas et al. to a wide variety of applications, particularly irregular shapes, is thus limited. The extruded mass may be broken up and put through the process again. That is, according to Thomas et al., it may be reformed.
Childs et al. U.S. Pat. No. 4,397,622 discloses a friction-actuated “Conform” process in which loose particulate copper or aluminum is extruded under pressure to form elongated solid objects.
In friction welding, frictional forces may be used to bond together components made, for example, of dissimilar materials. U.S. Pat. No. 5,697,545 to Jennings et al., disclose a friction welding operation in which oscillatory relative movement between two solid components under load generates enough heat to weld the two solid components together. Powders are not involved.
In friction surfacing operations, frictional forces are used to clad the surface structure of an article. For example, U.S. Pat. No. 5,183,390 to Amos and U.S. Pat. No. 4,930,675 to Bedford et al. teach friction surfacing with material derived from a solid consumable friction rod. The solid rod or probe has sufficient strength to withstand the thermal and mechanical forces imposed upon it. The composition of the facing is determined largely by the composition of the rod. In conventional friction welding and friction surfacing operations, frictional forces are relied on to generate heat in an amount sufficient to locally plasticize solid materials.
In a different art, but also known, is powder metallurgy. In powder metallurgy, net or near-net shape articles are conventionally produced by hot or cold pressing loose metallic powders, often with high temperature steps where the temperatures are at or above the melting points of at least some of the constituents. The metallic powders initially exist in a loose or non-coherent state. Typically, the required amount of raw material powder is placed within a casting or mold that is hydraulically pressed under heat. The powder is sintered under the compression and heat, and net or near-net shaped articles are formed. Undesired grain growth is often experienced. The heat is not frictionally generated. Utilizing powder metallurgy techniques to form articles is advantageous, particularly for difficult to work materials, or where physical properties are desired that are not obtainable by the application of other metallurgical processes. Capital equipment requirements are sometimes very substantial, as are processing costs. It is difficult and expensive to achieve substantially full density with many metallurgical powder consolidation processes.
These and other difficulties of the prior art have been overcome according to the present invention.